A planar antenna (antenna substrate) which transmits/receives radio waves in satellite communications, etc. has been proposed. A planar antenna has high directivity and transmits/receives radio waves on its front surface. In order to improve waterproof properties, impact strength, and rigidity, the planar antenna is covered by a radome comprised of thermoplastic resin (dielectric).
However, a part of the radio waves transmitted (radiated) from the front surface of the planar antenna propagate as surface waves along the surrounding radome. Therefore, the part of the radio waves propagate from the front surface of the planar antenna to the side surface or the back surface thereof. As a result, undesired radio wave radiation from directions (side surface side or back surface side) other than a main radiation direction (front surface side) of the planar antenna will increase, thereby deteriorating the directivity of the antenna. Especially in the case of satellite communications where directivity characteristics of the back surface side are regulated, it is possible that the directivity characteristics may exceed regulation values.